AR-223/556 a longer barrel may not do ANYTHING??? UPDATE post #82

This is a discussion on AR-223/556 a longer barrel may not do ANYTHING??? UPDATE post #82 within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; Here's the story. I wanted a coyote AR, meaning a gun that would shoot 53-55 gr ammo at higher velocity. I wouldn't mind having a ...

AR-223/556 a longer barrel may not do ANYTHING??? UPDATE post #82

Here's the story. I wanted a coyote AR, meaning a gun that would shoot 53-55 gr ammo at higher velocity. I wouldn't mind having a little more accuracy while I'm at it. Here's what I did and the surprising, make that disappointing results I got.

First I found a Shilen SS 20" heavy barrel with a 1 in 8 twist - a matching bolt comes with it. That seemed pretty good to me, the twist rate should stabilize anything from 53 gr up to 75 gr. The quality of the barrel should improve accuracy and according to two studies I've seen lately, one posted by Jon in my 300 BO thread, and another from a very meticulously done study measuring the effect of barrel length on the M855 round. That particular study showed a MV of 3000 fps for the 20" bbl and 2800 fps for a 16" bbl.

The table that Jon provided was based on the XM193 and listed the MV for the 20" bbl at 3247 fps and 3083 fps for the 16" bbl. So the study indicated an increase of 200 fps for the XM855 and the table indicated an increase of 164 fps.

So I was expecting an increase close to 200 fps. Know what I got? About 80 fps!?!? That's no where near what the two references I mentioned indicated. Actually, I didn't have any 55 gr at the time, so I chrono'd what I had, and what I had was XM855, Federal Fusion, and Hornady Superformance 75 gr BTHP. here are the results:

bbl/load

XM855

Hornady 75 gr

Fusion

16" bbl

2998

2677

2723

20" bbl

3057

2654

2791

The velocities are averages of five shots. Here's the breakdown:

XM855 - spec'd at about 3000 fps:
The 20" bbl produced a 2% higher velocity than the 16" bbl - that's about a 60 fps increase or a 15 fps increase per inch of bbl. That's a far cry from what the two references indicated.

Federal Fusion 62 gr - spec'd at 3000 fps:
The 20" bbl produced a 2.5% higher velocity than the 16" bbl - that's about a 68 fps increase or a 17 fps increase per inch of bbl. Again, a ling way from what the two references indicated.

Soooo, I'm disappointed and baffled. I would have never believed such dismal results from increasing the barrel length by 4 inches. Twenty inches is a 25% increase from the 16" barrel and the best increase in velocity was 1/10 of that - 2.5%. And in one case, the 20" was actually slower than the 16".

This seems to go against all prevailing expectations - any thoughts about why the 20" bbl did so little when other references are indicating over twice the values I got???

Guys I would appreciate any thoughts on this. I have this nice, expensive, heavy barrel that seems to be of little benefit. The accuracy issue is still 'on trial' but I'm not seeing any improvement in accuracy over my Diamondback AR with a 16" barrel with the grenade launcher cut in it.

I'm no expert, and am really throwing a dart here, but perhaps velocity will increase as the barrel breaks in more. I've read that until the throat and rifling is "polished" you may see lower velocity.

Yeah buddy, the longer barrels don't do what people expect. They're no more accurate than the same barrel cut shorter and in some cases don't provide much more velocity to make it worthwhile.

Some ammo will only have a difference of 100fps between a 16" and 20" barrel. It's not so much the length, but the percentage you cut that makes the most difference. 2" off of a 20" barrel isn't much, but 2" off of a 10" barrel is huge in terms of velocity.

I know some guys using 24" barrels for longer/unknown range prairie dog and coyote hunting, but for me, a 14.5" or 16" works fine, though I wouldn't mind a stainless precision 16" upper eventually for improved accuracy.

5.56 needs a lot of barrel to burn up the powder. That's why the concussion from an 11.5" is so much more than a 9" 300blk, which requires about half the barrel length of 5.56 to get full powder burn. Other than that, I see no reason to go with a 20" barrel over a 16" unless someone just loves the rifle length gas system.

Imagine the difference if you were using 75gr TAP or 70gr TSX. The velocity difference would probably be a good but less than the 80fps.

Here's some velocity numbers from the Black Hills 50gr TSX with different barrel lengths from Doc Roberts.

I added the distance after the velocity, which is where it reaches the expansion threshold (2,300fps) of 50gr TSX. Something like m855 and m193 reaches it at much shorter distances, or in some cases, doesn't reach it at all like with an 8" barrel and only a few yards with the 10.5".

Proven combat techniques may not be flashy and may require a bit more physical effort on the part of the shooter. Further, they may not win competition matches, but they will help ensure your survival in a shooting or gunfight on the street. ~Paul Howe

Yeah, I'm not really surprised at all by your results. I read an article a while back that basically busted every conventional thought regarding longer barrels. Had I known then, what I know now, I'd probably gone with another 16" for my predator/pest/hog/anything else scoped rifle.

With the M855 62 grain round a 20" barrel gives you the maximum return on velocity in the 5.56 NATO rounds. Longer and it actually decreases, shorter and the velocity only drops very modestly....about 50 fps per inch of barrel until about 11", then it begins to drop precipitously.

is done with the XM855. They start with a 24" barrel and cut and re-crown in one inch increments. They see a change in velocity of 200 fps going from 20" to 16". My results only show a 60 fps change??? I can understand if I got somewhere close to 200 fps, but I'm not even in the ball park.

Here's the other thing, how is Hornady getting 2930 fps from the 75 gr? From my chronograph a 16 and 20 inch bbl won't come close to that value.

It's kinda like your buddy and the 24" barrel. Now, having seen what I've measured first hand, I wonder if these coyote hunters are really getting anything out of their longer barrel. Like you said, and I'll add: especially at these barrel lengths (16" - 24") they don't gain accuracy, well, unless the change to a longer barrel involves a better quality barrel as well, but they aren't getting it from the length.

With the M855 62 grain round a 20" barrel gives you the maximum return on velocity in the 5.56 NATO rounds. Longer and it actually decreases, shorter and the velocity only drops very modestly....about 50 fps per inch of barrel until about 11", then it begins to drop precipitously.

Yep, that's the exact study I referred to in my OP. But notice in that study, they got a decrease of 200 fps going from a 20" bbl to a 16" bbl - I got only a 60 fps increase. Something ain't right here somehow.

Yeah, I'm not really surprised at all by your results. I read an article a while back that basically busted every conventional thought regarding longer barrels. Had I known then, what I know now, I'd probably gone with another 16" for my predator/pest/hog/anything else scoped rifle.

I hear ya! OTOH, I've seen quite a few reports of chrono results indicating something between 30 - 50 fps per inch of barrel increase/decrease. I'm showing about 15 fps at best.

I'm like you - had I known this would be the result, I would have just gone with a better 16 inch barrel.

Could be that they were using a single shot (note no gas tube). Also realize that you are using different ammo (different bullet/different powder) that may and likely does affect burn rate/pressure and thereby velocity.

Again, I selected mine for barrel quality and rifle length gas first, then with an eye on velocity.

18" seemed to me to be the best compromise, but I also want to shoot beyond 400 yards. If it was going to be 400 or less, a quality mid length gas 16" barrel would suit me fine.

BTW: "extra" was a tongue in cheek..... I haven't chrono mine, but my main concern is accuracy, not velocity.

My accuracy seems to be determined more by my skill (or lack thereof) than the equipment I am using.

Velocity? What difference does a few feet per second really make?

An average or so shooter may be limited by his skill, but a superb shooter is limited by his equipment. I'm not saying I'm a superb shooter, but if a longer barrel is also a more accurate barrel, I get both.

I was expecting a lot more than a few feet per second, more like 200 fps. At 300 yards that 200 fps makes a huge difference in bullet drift due to wind, how far it will drop, and how much energy it will retain.